Technical details aside, the problem is caused by the placeholders for signatures and addresses as their width grows when the size of the font increases.
To see what I mean, add the following line to your code before convertURI() and you will notice that some placeholders exceed the right margin of the text body in the generated pdf.
client.setPdfScalingFactor(300);
The solution is to make the placeholder width smaller and constant, independent on the used font size.
Hope this helps.